I found if you stand in the doorway where the ammo box is and shoot through the broken chopper. you have to aim it just right or you will hit the chopper. It takes 3 of langs rockes to kiIll the tank. If you run out of ammo just go to the ammo box and get more rockets. I had a heck of a time trying everything to kill this tank. I just beat the mission on extream doing this. hope this helps